The present invention relates to an apparatus for molding a plastic seal inside a closure for closing a container. The apparatus is particularly suitable for molding an annular seal inside a closure with a so-called "twist-off" opening.
Devices for molding a seal inside a closure are known for example from European Patents 37,651 and 462,513. In these devices, the seal is produced by depositing a dose of plastic material in the pasty state inside the closure and by compressing said dose with a punch, so as to form a layer which covers the bottom of the closure. Conventional devices are suitable for small closures, such as for example crown caps; in the case of much larger closures, the described system has limitations caused by the difficulty of forming a uniform layer, also in view of the fact that an annular expansion, against which the rim of the container must be hermetic, is generally required along the peripheral region of the closure.
Moreover, the pressure required to compress the material until it covers the bottom of the closure becomes excessive and can cause deformations or damage to the closure. Accordingly, seals in large closures are manufactured with devices which work with plastic materials in the semifluid state which, after being deposited inside the closure, is spread so as to cover the bottom by spinning the closure. However, these devices have a very low capacity and have limitations as regards the use of certain plastic materials, particularly thermoplastic materials. Moreover, it is not possible to produce seals having a differentiated cross-section.